Guidance Notes for 1:50,000 Colour Raster

Introduction

These guidance notes provide guidance on the appearance of maps created from the 1:50,000 Colour Raster data and on the data itself.

1:50,000 Colour Raster images mirror the Ordnance Survey Landranger® map series and are used in Digimap to create Local View maps.

Data Format

1:50,000 Colour Raster data, as its name suggests, is a raster dataset. The 1:50,000 Colour Raster data is image data and so individual features cannot be removed from the dataset. Each tile covers 20km x 20km and there are 812 edge matched tiles which cover Great Britain. This includes the Isle of Man.

Digimap delivers the 1:50,000 Colour Raster data tiles as compressed TIF (Tagged Image File Format) files. These are 256 colour 8-bit LZW compressed TIFs. Each tile is accompanied by a TIF World File, with the extension .tfw.

The .tfw file (TIF World Files), contains information that can be used by some Geographical Information Systems (GIS) to georeference the corresponding TIF image associated with the .tfw file. This information is important when combining the 1:50,000 Colour Raster image data with data from other sources. The .tfw files supplied by Digimap are sourced from the Ordnance Survey.

The .tfw file will look something like this:

5.00000000000000
0.00000000000000
0.00000000000000
-5.00000000000000
320000
520000

In order to make use of the .tfw file most GIS packages will require the .tfw to have the same filename as the TIF file it references. It should also be located in the same directory as the source file. e.g. tl60.tfw provides the georeferencing information for tl60.tif

The TFW file is a simple 6 line ASCII file. The table below explains what each line of information represents.

Line Explanation
1 X resolution: dimension of a pixel in the X direction
2 The magnitude of translation
3 The magnitude of rotation
4 Y resolution: the negative of the dimension of a pixel in the Y direction
5 The X ground coordinate of the upper left pixel
6 The Y ground coordinate of the upper left pixel

Download Constraints

Data Download will only permit 25 raster tiles to be downloaded at once because of their size. Each TIF is quite large even when compressed and therefore will require larger amounts of disk space than other vector format data tiles. Each Landranger® map comprises approximately four raster data tiles.